High-frequency amplifier neutralization circuits



May 1, 1951 w. R. KOCH 2,550,930

HIGH-FREQUENCY AMPLIFIER NEUTRALIZATION CIRCUITS Filed Jan. 10, 1946 T0SOURCE 0F SIGNALS L0 2 E CIRCUIT 70 SIGNAL GRIDS OF PRIOR CUNIROL- IEOAMPLIFIERS4 +5 r0 AVC RECTIFIER INVENTOR mur/sw R. we

ATTORNEY Patented May 1, 1951 HIGH-FREQUENCY AMPLIFIER NEUTRAL- IZATIONCIRCUITS Winfield E. Koch, Haddonfield, N. J assignor to RadioCorporation of America, a corporation of Delaware Application January10, 1946 Serial No. 640,286

2 Claims- (Cl. 179-171) My present invention relates to novel andimproved circuits for neutralizing high frequency amplifier tubes of thescreen grid type, and more particularly to circuits for providing morestable intermediate frequency (I. F.) amplification at higher signalfrequencies.

While at relatively low radio frequencies amplifier tubes of the typeusing a screen grid have negligible regenerative feedback from theplate, or anode, to the signal input grid through the residual orinherent plate to input grid capacity, yet at higher radio frequenciesappreciable regeneration takes place through the residual plate to inputgrid capacity. The magnitude of the regenerative feedback depends onvarious factors, such as the gain through the tube, the value of theresidual capacity, the size of the tuning capacities and the signalfrequency. Feedback through the residual plate to input grid capacity isincreased where small capacities are used in the transformer tunedcircuits, as may be done where compensation is included for changes intube input capacity caused by changes in tube voltages.

AVC (automatic volume control) bias, negative in polarity, causesundesirable changes in input grid to cathode capacity (tube inputcapacity) thereby to affect the tuning of the amplifier. In the past ithas been proposed to employ an unbypassed, or degenerative, cathoderesistor in such an amplifier circuit thereby permitting the use of asmaller input capacity with resulting increase in the gain of eachstage. However, the plate to input grid capacity then causes increasedcoupling from the plate to the grid circuit, with subsequent increase ofregenerative feedback.

The selectivity characteristic of the amplifier is altered and becomesunsymmetrical, and even varies with change in value of the AVG voltage.This presents an amplifier design problem, as, for example, in the caseof I. F. amplifiers of frequency modulation (FM) receivers operating at8 megacycles (mc.) per second.

In accordance with my present invention, I provide a simple andeffective method of substantially reducing, if not eliminating, theundesirable capacity coupling between the input grid and output platecircuits by resonating in effect the undesirable capacity with aninserted inductive reactance at the operating signal frequency.

Another important object of my present invention is to provide a novelmethod of decoupling a source of high frequency energy from a loadcircuit, by in effect resonating a coupling capacity between the sourceand load to the frequency of the high frequency energy.

Another object of my present invention is to eliminate the eifect ofundesired plate to input grid capacity in an amplifier of the screengrid type by utilizing mutual inductance existing between the bypasscapacitor leadson the ground side of bypass capacitors for seriesresonating the undesired capacity to the operating frequency of theamplifier.

A more specific object of my invention is to provide neutralization ofundesired plate to input grid capacity of an amplifier tube over a Wideband of signal frequencies.

Still other objects of my invention are to improve generally thestability of very high frequency amplifiers, and more especially toprovide simple and effective neutralization for I. F. amplifiersoperating in the megacycle range.

Still other features of my invention will best be understood byreference to the following description, taken in connection with thedrawing,

in which I have indicated diagrammatically several circuit organizationswhereby my invention may be carried into effect.

In the drawing:

Fig. 1 is a schematic diagram of a circuit embodying an embodiment ofthe invention;

Fig. 2 is a simplified diagram explanatory of the circuit shown in Fig.1;

Fig. 3 is a schematic diagram of a modification of the circuit shown inFig. 1;

Fig. 4 is a schematic diagram of another modification of the circuitshown in Fig. 1;

Fig. 5 is a schematic diagram of a further modification of the circuitshown in Fig. 1;

Fig.6 is a simplified diagram explanatory of the circuit of Fig. 5; and

Fig. 7 is an equivalent circuit diagram of the circuit of Fig. 6.

Referring now to the accompanying drawings,

wherein like circuit elements are represented by.

similar reference numerals, the signal amplifier tube I is generally ofthe type embodying a positively charged screen grid electrode. The tubemay be a tetrode, a pentode, or any other multigrid tube utilizing ascreen grid. As shown in Fig. 1 the tube I comprises a cathode 2, asignal input electrode 3, a screen grid 4, a plate or anode 5, and asuppressor grid 6 located between screen grid 4 and plate 5. The signalinput network may be coupled to any suitable signal source. By way ofspecific example for the purposes of the present application, it isassumed that the numeral i denotes an intermediate frequency (I. F.)input transformer whose primary circuit 8 and secondary circuit 9 arerespectively tuned to a desired I. P. value. The primary circuit 8 maybe coupled to the output electrodes of a prior I. F. amplifier tube, orit may be located in the plate circuit of a converter tube. In eithercase it is assumed that the tube i is utilized in a superheterodynereceiver system, a type of signal re ceiving system which issubstantially universall' employed today in radio communication.

The receiving system may be constructed either to receive amplitudemodulated carrier waves, frequency modulated carrier waves or phasemodulated carrier waves. The problem sought to be solved by the presentinvention arises in the radio frequency ranges substantially higher thanthe present amplitude modulation broadcast band of 550 to 1700kilocycles (kc.) per second. For example, when operating the receiver IFin the megacycle (mc.) ranges the problem of plate to signal input gridfeedback becomes appreciable regardless of the character of themodulation.

Although the invention is applicable to signals in an extended range ofhigh frequencies, the circuit of Fig. 1 is intended for intermediatefrequencies of the order of 8 megacycles per second. The cathode 2 oftube I is shown connected to ground by an unbypassed resistor H2. Thesignal input grid 3 is connected to the high alternating potential sideof the resonant secondary circuit Sywhile the low potential side ofcircuit 9 is returned to ground through the AVG line H. Those skilled inthe art of radio communication are fully aware of the construction ofthe AVG system, and will know that a filter resistor 52 connects theinput circuit 9 to the AVG line H to suppress alternating currentvoltage components in the AVG voltage supplied from a suitable AVCrectifier. For the purposes of the present application it is believedsufficient to show the AVG line H, and to indicate that it is connectedto a source of negative direct current voltage which is derived from theload resistor of an AVC rectifier.

In general, the AVG rectifier consists of a diode, or any other suitabledetector, supplied with amplified signals before they are demodulated.The rectifier load resistor which is connected to develop negativedemodulated signals with respect to ground, the developed signalsbecoming increasingly negative with an increase of carrier amplitudeabove a predetermined carrier level. The AVC voltage is taken from asuitable portion of the load resistor and supplies the negative voltageto the signal grids of the various controlled signal amplifiers. It isaccordingly indicated in Fig. 1 that AVC line i i may be connected tothe signal grids of prior controlled amplifiers, in addition to beingconnected to the signal grid 3 of tube I. The function of the AVG systemis to maintain a substantially uniform carrier level at the demodulatorregardless of relatively wide amplitude variations at the signalcollecting device of the receiver. This is accomplished by varying theeffective negative voltage or bias of signal grid 3, as well as othercontrolled signal grids. It is to be understood that the AVG line Hreturns to ground through the load resistor of the AVG rectifier. Thedirect current voltage drop appearing across resistor 59 as explainedbelow is therefore also applied in a negative polarity sense to thesignal grid 3.

Plate of amplifier tube I is connected to the high alternating potentialside of the resonant primary circuit I 3 of the I. F. output transformeri l. The low alternating potential side of circuit is is connected tothe positive terminal (B+) of a. suitable direct current energizingsource through the resistor E5. The other terminal of this source asgrounded and the low potential side of the circuit I3 is bypassed toground by the I. F. bypass condenser l8; Similarly the I. F. bypasscondenser ll bypasses I. F. currents from the low potential side ofinput circuit 9 to ground. In accordance with my present invention thebypass condensers i6 and H are both connected in common to groundthrough an inductive element 18.

The secondary circuit IQ of the I. F. output transformer M may beconnected to any suitable signal output circuit. For example, the outputcircuit may include a further I. F. amplifier, or it may consist of thedemodulator of the receiving system. It is to be understood that each ofcircuits l3 and i9 is tuned to the operating I. F. value. Furthermore,for relatively narrow band reception, as in the case of amplitudemodulation and frequency modulation reception, each of transformers land M is arranged to be substantially band-pass in character. In orderto preserve the band-pass selectivity of the selector circuits coupledto the input electrodes and output electrodes of tube the cathode biasresistor I0 is kept free of capacitive bypassing.

The operation of the amplifier tubes depends upon the emission ofelectrons by the cathode 2 and their passage to and collection at theanode 5. This electron flow is controlled by the instantaneous voltageof the control grid 3 so that direct current from the 13+ terminal toground (the plate current) undergoes variations. These plate currentvariations develop across resistor it, between cathode and ground, asmall signal voltage in phase with the incoming signal voltages betweenthe control grid 3 and ground. As the bias of the control grid ischanged, the amplification of the tube changes and so does themeasurable capacitance between this grid and the cathode, represented at20. However as the tube gain is changed the signals across resistor l0change in amplitude and if the resistance is suitably chosen, thesignals carried capacitively from the control grid to the cathode can bemade to remain substantially independent of the tendency of capacitance20 to vary.

The efiect is to prevent the AVG bias from affecting the selectivity ofthe input circuit 9. However, the very introduction of the unbypassedresistor ill gives rise to a feedback problem by virtue of the fact thatthe effect of the plate to signal grid capacitance becomes moreimportant.

This can be seen by considering the various other inherentinter-electrode capacities of tube I. These undesired inter-electrodecapacities 2|, 22 are shown in dotted lines to indicate that thecapacities exist inherently in the tube circuit and between therespective electrodes. Thus, the signal grid to cathode capacity is inshunt across the resonant input circuit 8, and affects the frequency ofinput circuit 9. By using the unbypassed resistor is, the smallereffective changes in capacity 29 do not require the diluting action oflarge capacitances in input circuit 9 and enable operation with theimproved gain of low capacitance inputs. However, because of the factthat the overall input capacity is made smaller, the coupling due toinherent capacity 2! between the plate 5 and signal grid 3 becomeseffectively larger. In other words, there is provided an appreciableregenerative feedback path from the output circuit of tube to its inputcircuit. The numeral 22 indicates the output capacity ofthe tube, or theinherent capacitance existing between plate 5 and cathode 2.

Depending upon the gain secured with tube I, the frequency of operation,and the magnitude of the degenerative resistor Hi, the regenerativefeedback through capacitance 2| may be sufficient to cause theselectivity characteristic at I. F. input transformer 1 to becomesubstantially un-, symmetrical. Further, the shape of the characteristicwill change with variation in magnitude of the AVG bias. Hence, it isseen that even though the amplifier tube employs a screen grid which, atlower radio frequencies, acts to reduce the capacitance 2| to aninappreciable value, yet at higher frequencies, by virtue of thedesirable reduction of input capacitance 29 the undesired capacitance 2|assumes a sufiicient'magnitude to cause instability in the I. F.amplifier stage.

In accordance with my present invention the instability caused by theundesired capacitance coupling 2| is substantially eliminated by asimple and effective device. The inductiveelement l3 produces asubstantial decoupling between the input andoutput circuits 9 and I3respectively. This decoupling occurs by virtue of the fact that theinductive reactance of element I8 coacts with 1 the capacitancereactance of the inherent capacitance 2| to produce the equivalent of aseries resonant path common to the input and output circuits 9 and l3respectiv'ely, the series resonance occurring at the signal frequencywhich in this case has been assumed to be 9 me. By virtue of theeffective common series resonance path there is substantially nocoupling possible between the input circuit 9 and the output circuit l3.Hence, the feedback through the capacitance 2| is greatly reduced, ifnot eliminated.

In order to provide a clearer understanding of the operation of myinvention I have depicted in Fig. 2 a simplified equivalent circuitdiagram of the I. amplifier circuit shown in Fig. 1. It can bedemonstrated that the tube capacities 20, 2|, and 22 of tube I in Fig. Iexist in the nature of a bridged T network as depicted in Fig. 2. For adetailed explanation of how the transformation is secured, reference ismade to the book entitled Transmission Circuits for TelephonicCommunication by K. S. Johnson, copyrighted 1927 by D. Van Nostrand(30., Inc., appendix D, page 282, Figs. 28A, 28B. In Fig. 2 only theinput circuit and output circuit are shown, the high potential sidesbeing connected through the series arranged capacities 29' and 22',while the low potential sides of circuits 9 and I3 are connected throughthe series arranged bypass condensers l1 and IS. The inductive elementH3 is shown connected from the junction of condensers H and 15 toground, while the capacitance 2 is connected from the junction ofcondenser 29 and 22' to ground. The capacitance 2| and inductance la ineffect provide a series resonant path tuned to the operating I. F.Value. This resonant path 2|, l8 provides the means for decoupling theinput circuit 9 and the output circuit IS. The ground connection at theupper end of coil l8 does not interfere with the decoupling effect ofpath l8, 2|, because no other grounds exist on the output and inputcircuits. Accordingly, no radio frequency current will flow through theground connection.

Substantially zero coupling exists between the circuits 9 and [3 of Fig.2 due to the existence of the effective series resonant path 2 H3. The

actual values of the equivalent network can be calculated from theinformation given .in the Johnson publication cited above. On page 189,Fig. 5, sub-figure '8 and 5A, the transformation for the network,including the inductance, is

shown. For the purposes of the present application it is sufficient topoint out that inserting the inductive element |8 inthe common path toground from the junction of bypass condenser l1 and I 6 will effectivelyprevent undesired coupling between the output circuit l3 and inputcircuit 9. In Fig. 3 I have shown a simple and economical scheme forproviding inductive reactance l8. The-magnitude of inductancerequired'at 3 mc., for example, is about as much as that of a two inchlength of wire. Thus, in Fig. 3 the-bypass capacitors l1 and I6, shownsymbolically as rectangles, have a common connection 8 to the groundedend of cathode resistor Hi. The inductance I8 is shown dotted toindicatethat it is the inductance provided by short lead |8'. The value:of inductance I8 can then be adjusted by shortening .101 bending. of thebypass capacitor leads if a close adjustment is desired, but, in gen:-eral, the 'valueof inductance l8 will not be very critical.

In Fig. 4 tube is a ;tetrode of the screen grid type. The cathode isshown connected to ground through a suitably bypassed bias resistor, I0,and the low potential side of the input circuit 9 is connected directlyto the grounded end of bias resistor Hi. It will be noted that. the AVGcircuitis dispensed with. In other words, the amplifier circuit shown inFig. 4' does not utilize AVC,]and the grid circuit is, therefore,returned directly to the ungrounded end of the inductance element I8. Ihave shown the inductive element I8 provided by the inductance of theshort lead 3|, and for this reason the inductive element I3 isrepresented by dotted line across lead 3|. In this modification theinductive reactance of inductive element l8 will be chosen so that itseries resonates the undesired capacitance 2| to the operating frequencyof circuits 9 and I3 thereby substantially eliminating coupling betweenthese latter two circuits.

In the case of an amplifier adapted to have applied to its input circuita wide band of high frequencies, as for example in the case of anamplifier of video modulated carrier waves, there may be employed amodification as shown in Fig. 5. In this modification the circuitelements are generally as shown in Fig. 3, except that a condenser 50 isshunted across the short lead 40 located between the grounded end ofbypassed resistor l9 and the junction of the leads 4| and 42. Thenumerals it, I! and 59 designate schematic representations ofcondensers.

In Fig. 6 I have shown the equivalent circuit diagram of the circuit ofFig. 5. It will be noted that the equivalent T network consisting oftube capacities 20, 2|, and 22' include additional elements between theground side of condenser 2| and the common low potential sides ofcircuits 9 and'|3. This additional network consists of the inductiveelement l8 which is shunted by a series combination of condenser 59andinductive element 5|. This inductive element 5| is effectivelyprovided by the inductance of the leads of capacitor 50.

In Fig. '7 I have shown the equivalent circuit diagram (transformation)for the circuit shown in Fig. 6 including the shunt network It, 2|, 59and 5|. It will be seen that the complex network is equivalent to a pairof series resonant circuits which are staggered in tuning. Thistransformation to the equivalent circuit of Fig. 7 is shown in theJohnson publication, Appendix D, Figs; 13C and 13D. Thus, the condenser10 and coil H provide one series tuned .circuit, while the condenser 80and inductance 8| provide the second series tuned circuit. The resonantfrequencies of these series tuned circuits are spaced so as to providesubstantially zero coupling over the wide band of signals to betransmitted.

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention.

"What I claim is:

1. In a high frequency signal amplifier of the I type provided with anelectron tube including at least a cathode, a signal input grid, ananode, and a positive screen grid between the signal grid and anode: ahigh frequency signal input circuit coupled to said signal input grid; ahigh frequency output circuit coupled to said plate; a resistorconnected between the cathode and a point of fixed reference potential;an inductive element connected in a common circuit path between saidcommon return conductor and the low potential sides of said input andoutput circuits; and a capacitor connected in shunt with said inductiveelement to provide substantial decoupling between the input and outputcircuits over a relatively wide band of signal frequenciesnotwithstanding the coupling between these circuits via the inherentcapacitance between the input grid and the plate.

2. In a high .frequency signal amplification system: an amplificationstage including a cathode and a control grid; an input circuit and anoutput circuit, each having a high signal poten- ,tial lead and a lowsignal potential lead; the high signal potential lead of the inputcircuit being connected to the control grid and the high signalpotential lead of the output circuit being connected to the anode; aresistor connected between said cathode and ground, a first capacitorand a second capacitor being connected in series arrangement between thelow signal potential leads of said input and output circuits, and aninductive element being connected between the junction of saidcapacitors and ground, and a capacitor connected in shunt with saidinductance.

WINFIELD R. KOCH.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

